Three dimensional image projector stabilization circuit

ABSTRACT

A projector system that includes a first input device, a second input device, a control device, a sensor and a phase locked loop (PLL). A phase reference signal is created based on a signal rate of the first input device. A phase feedback signal is created based on the rotational speed of the second input device as it is measured by the sensor. The PLL compares the phase reference signal and the phase feedback signal to determine whether the first input device and the second input device are synchronized. A signal is sent to the control device for the second input device to change the rotational speed of the second input device in response to determining that the first input device and the second input device are not synchronized.

BACKGROUND

The present invention relates to a stereoscopic three dimensional imageprojector, and more specifically, to a feedback circuit for a threedimensional projector.

Three dimensional (3D) movies and pictures have become a popular form ofentertainment due to the increased realism of the images. 3D imagesutilize the human physical trait of binocular vision. Human eyes arespaced about 2 inches (5 centimeters) apart; therefore each eye sees theworld from a slightly different perspective. The brain receives bothimages and has a binocular vision function that correlates thedifference between what each eye sees to determine distance. Thedetermination of the distance provides the 3D effect that a person sees.

To create a binocular image on a two dimensional surface (2D), such as amovie or television screen, the user typically wears glasses. Theglasses alter the way that the user views the images to create thesimulated 3D effect. Typically there are two types of glasses, passiveglasses and active glasses. The type of glasses used will depend on thetype of image projection system being used.

Passive glasses rely upon an optical effect created by using differentlenses for each eye. The projection system emits a sequential series ofimages where subsequent images are slightly offset. The images arearranged such that the user sees the first image through a first lens ofthe glasses (e.g. the right eye) and the second image is seen with theother lens (e.g. the left eye). Since the images are projected quickly,the user does not notice the multiple images, but rather sees a threedimensional effect. With active lenses, the glasses wirelesslycommunicate with the projector to synchronize the operation of theglasses with the images being displayed. With active glasses, the lensesare typically liquid crystal displays (LCDs) that can switch betweentransmitting light and blocking light. In this way, the glasses mayrapidly switch the left and right lenses between clear and opaque. Whilethe glasses are switching, the television is projecting a series ofsequential images. When this switching is synchronized between thetelevision and the glasses, the user experiences a three dimensionaleffect.

In 3D projectors using both active and passive lenses, synchronizationof the images is critical to the functionality of the projector. Becausethe multiple images projected typically have different polarizations, itis imperative that the light source, imaging device, and polarizationmodulator within the projector remain synchronized. If these devices arenot properly synchronized, the images will not be correctly polarized tocreate the 3D effect.

SUMMARY

An embodiment is a projector system for a portable electronic devicehaving a first input device, a second input device, a control device, asensor and a PLL. The projector system is configured to perform a methodthat includes creating a phase reference signal based on the signal rateof the first input device. A phase feedback signal is created based onthe rotational speed of the second device as it is measured by thesensor. The phase reference signal and the phase feedback signal arecompared by the PLL to determine whether the first input device and thesecond input device are synchronized. If the first input device andsecond input device are not synchronized, the PLL sends a signal to thecontrol device of the second input device, and the control devicechanges the rotational speed of the second input device in response toreceiving the signal.

A further embodiment is a system that includes a control device thatcommunicates with a second input device. The control device isconfigured to control the rotational speed of the second input device.The system also includes a sensor that communicates with the secondinput device and is configured for determining the rotational speed ofthe second input device and outputting a phase feedback signal based onthe determined speed of the second input device. The system furtherincludes a PLL that is configured to receive and compare a phasereference signal from a first input device and the phase feedback signalto determine whether the first input device and the second input deviceare synchronized. If the PLL determines that the first input device andthe second input device are not synchronized, it sends a signal to thecontrol device to change the rotational speed of the second inputdevice.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a schematic view of an exemplary three dimensional (3D) imageprojector in accordance with an embodiment of the invention;

FIG. 2 is a flow chart for a method of operating a feedback circuit in a3D image projector in accordance with an embodiment of the invention;

FIG. 3 is a schematic view of another exemplary 3D image projector inaccordance with an embodiment of the invention;

FIG. 4 is a top schematic view of the 3D image projector of FIG. 3; and

FIG. 5 is a flow chart for a method of operating a feedback circuit in a3D image projector in accordance with another embodiment of theinvention.

DETAILED DESCRIPTION

An embodiment of the present invention includes an electronic feedbackcircuit for synchronizing polarization modulation elements with flashinglight sources or data modulation devices in stereoscopic threedimensional (3D) pico projectors. In an embodiment, a 3D stereo inputsignal (such as from an electronic stereo jack or similar system) isderived from the frame rate of a digital image system and used as aphase reference to synchronize the input signal with a polarizationmodulator. In other embodiments, the same approach is used tosynchronize various elements within a pico projector, including elementssuch as the frame rate of the signal driving the image system (a liquidcrystal on silicon display or “LCoS display”, a digital mirror device or“DMD”, or a similar device), a rotating polarization modulator, aflashing light emitting diode (LED) or laser light source, or multiplelight sources with a common dichroic combiner.

One of the two devices to be synchronized provides the stereo jackinput, and the other provides a signal from a modulation sensor (forexample, a tachometer measurement of the rotating polarization element,or a fraction of the modulation signal driving the LED or LCoS device).In an embodiment, a modulation sensor input is delayed by some amountdue to variations in the circuitry layout for different pico projectordesigns. To compensate for this delay, embodiments incorporate avariable quiescence delay that cancels out the delay in order to achievethe necessary synchronization accuracy. In an embodiment, the feedbackfrom the modulation sensor is amplified and conditioned prior to drivinga phase locked loop (PLL) which also uses the phase reference signal(divided by 2) to account for the fact that the signal must be modulatedat twice the speed of the polarization element.

With reference now to FIG. 1, an exemplary projector is shown forprojecting a 3D image from a single projection lens. The projector 20includes a first light source 22 and an opposing second light source 24.The light sources 22, 24 are arranged to direct light towards eachother. Each light source includes three monochromatic LED's: a red LED30, a green LED 32 and a blue LED 34. The LED's 30, 32, 34 are arrangedto form three sides of a square and direct light toward the center oflight source 22, 24. Each LED 30, 32, 34 may be coupled to direct lightinto a light collection optic 36.

The light collection optic 36 directs the light from the LED's 30, 32,34 into a dichroic color combiner 38. The dichroic color combiner 38combines light from the LED's to create a desired light color. The lightfrom the first light source 22 exits via an open side 40 and passesthrough a fly's eye lens 42 and a pre-polarizer lens 44. The light exitsthe pre-polarization lens 44, in the direction of arrow 26, and passesthrough a focusing lens 52 that focuses the light into a polarizing beamsplitter (PBS) 54. The second light source 24 operates in a similarmanner such that the light emitted from the LEDs exits an open side 46and passes through a fly's eye lens 48 and a pre-polarizer lens 50.After being conditioned by the fly's eye lens 48 and the pre-polarizerlens 50, the light travels in the direction shown by arrow 28, through afocusing lens 56 before entering the PBS 54.

A PBS 54 is an optical component that splits incident light rays into afirst (transmitted) polarization component and a second (reflected)polarization component. In the exemplary embodiment, the PBS 54 is adevice arranged to rotate about an axis 58. The PBS 54 has a surface 60that alternately reflects the light from the light sources 22, 24 as itrotates onto an imaging device 62. In the embodiment shown in FIG. 1,the imaging device 62 includes a LCoS display. The light reflects off ofthe surface 64 of the imaging device 62 with a polarization that thensubstantially transmits through the PBS 54, through the projection lensassembly 66 and out of the projector 20.

The projector 20 shown in FIG. 1 also includes a feedback circuit 100that is used to synchronize various components within the projector 20,particularly to stabilize polarization modulation. The feedback circuit100 is electrically coupled to communicate with the first light source22, the second light source 24, the PBS 54 and the imaging device 62.The feedback circuit 100 receives modulation signals from the PBS 54 andfrom the light sources 22, 24 or the imaging device 62, and then outputsa modulation signal to the PBS 54 to keep the PBS 54 synchronized withthe light sources 22, 24 or with the imaging device 62 during operation.Alternatively, there may be two or more feedback circuits 100, one ormore to keep the PBS 54 synchronized with the light sources 22, 24 andanother to keep the PBS 54 synchronized with the imaging device 62. Inother words, the feedback circuit 100 ensures that the PBS 54 isrotating at a speed such that the PBS 54 is in the correct position wheneither an image is displayed on the surface 64 of the imaging device 62and/or a light is emitted by one of the light sources 22, 24.

With reference to FIG. 2, an exemplary embodiment of a feedback circuit100 is provided for use in a projector for projecting a 3D image, suchas projector 20 is generally shown. As shown in FIG. 2, the frame rateof the imaging device 62 of FIG. 1 is being synchronized with therotation speed of the PBS 54 of FIG. 1. As shown in FIG. 2, the firstinput device being synchronized is the imaging device 62. A phasereference signal 73 is derived from the speed or frame rate of theimaging device 62. The phase reference signal 73 is input to a phaselocked loop (PLL) 88. In an embodiment, the whole phase reference signal73 is input to the PLL 88. In another embodiment, a fraction of thephase reference signal is input to the PLL 88.

The second input device shown in FIG. 2 is PBS 54 of FIG. 1. While theframe rate of the imaging device 62 is input into the feedback circuit100, input from the rotating PBS 54, is simultaneously gathered via asensor 80. In the embodiment shown in FIG. 2, the sensor 80 for the PBS54 is a tachometer that measures the rotational speed of the PBS 54. Inalternate embodiments, the sensor 80 detects the modulation signaldriving the imaging device 62. A phase feedback signal 81 that indicatesthe rotational speed of the PBS 54 is output from the sensor 80. Thephase feedback signal 81 output from the sensor 80 is adjustable via aprogrammable quiescence delay device 82 which is set based, for example,on how fast the PBS 54 is spinning. The quiescence delay device 82 is anoptional element of the feedback circuit 100 which may be required tocompensate for a delay that is introduced due to the design or circuitryof the projector 20. When the quiescence delay device 82 is used, a PBSmodulator driver 90 for the PBS 54 sends the quiescent delay device 82 aquiescence reference signal 84 that is based on the current rotationalspeed of the PBS 54 such that any delay in a phase feedback signal 81may be eliminated to achieve synchronization accuracy. Additionally, anoptional signal conditioner or amplifier 86 may be applied to the phasefeedback signal 81 that is output from the quiescence delay device 82prior to the phase feedback signal 81 being directed to the PLL 88.

The PLL 88 compares the phase reference signal 73 and the phase feedbacksignal 81 to determine whether the rotation of the PBS 54, issynchronized with the frame rate of the imaging device 62. Based on theresults of the comparison, the PLL 88 outputs a signal 92 to the PBSmodulator driver 90 causing the PBS modulator driver 90 to eitherincrease or decrease the rotational speed of the PBS 54. Thissynchronization of the PBS 54 with the imaging device 62 stabilizes thepolarization modulation occurring within the projector 20.

In another embodiment, the first input device is an LED, such as LED 30within light source 22 of FIG. 1. In this embodiment the frame rate ofthe LED 30 is being synchronized with the rotation speed of the PBS 54.In this embodiment, it may be necessary, depending on the projector 20,to adjust the phase reference signal 73 before it is input to the PLL88. For example, if the projector has two light sources, such as lightsources 22 and 24 as shown in the projector of FIG. 1, modification ofthe phase reference signal 73 is not required because each light sourceonly emits light when the rotating PBS 54 is in a given position. If theprojector has only a single light source, however, the light source willemit light twice for every rotation of the PBS 54. The phase referencesignal 73 must therefore be adjusted to correlate the emission of alight from an LED with the timing that a PBS 54 is in a given position.After any modification (e.g., by a reference signal modification block,not shown), the phase reference signal 73 is then directed into the PLL88. Alternatively, the difference may be compensated for with a delay inthe sensor 80.

With reference now to FIG. 3 and FIG. 4, a 3D projector 20 that includesa feedback circuit 174 is shown for projecting a 3D image from a singleprojection lens in accordance with an embodiment of the invention. Theprojector 120 includes a light generator 121 having three individuallaser light generators 123, 124, 125. In the exemplary embodiment, eachlaser light generator 123, 124, 125 includes a pair of monochromaticlaser diodes, with each of the pair of monochromatic laser diodes havingorthogonal polarizations relative to each other. In the exemplaryembodiment, the generator 123 includes a pair of red laser diodes 130,131, the generator 124 includes a pair of green laser diodes 132, 133and the third generator 125 a pair of blue laser diodes 134, 135.

The generators 123, 124, 125 are arranged in series. As a result, thediodes 130, 132, 134 are aligned in series to form a first light source122 and the diodes 131, 133, 135 are aligned to form a second lightsource 127. Each of the diodes 130, 132, 134 may include an integratedcollimator 129, 137, 139 that directs light toward one of adjacentdichroic mirrors 136, 138, 140. A dichroic mirror or filter usesalternating layers of optical coatings with different refractive indexesbuilt up upon a glass substrate. The interfaces between the layers ofdifferent refractive index produce phased reflections, selectivelyreinforcing certain wavelengths of light and interfering with otherwavelengths. Since unwanted wavelengths are reflected rather thanabsorbed, dichroic filters do not absorb this unwanted energy duringoperation which provides advantages in reducing heat when compared withan equivalent light filtering device since the filter will absorb energyall from all wavelengths except the desired color.

The mirrors 136, 138, 140 are each arranged to reflect the color oftheir respective laser diode 130, 132, 134. Further, the mirrors 136,138, 140 are disposed on an angle to reflect and blend the individualcolors to form white light. In the exemplary embodiment shown in FIG. 3,the first laser diode 130 emits a blue colored light 146 that reflectsoff of the dichroic mirror 136 towards the dichroic mirror 138.Simultaneously, the second laser diode 132 emits a green colored light148 that reflects off of the dichroic mirror 138 towards the dichroicmirror 140. The light 146 from the first laser diode 130 mixes with thelight 148 from the second laser diode 132.

Simultaneously with the emitting of light 146, 148, the third laserdiode 134 emits a red colored light 150 towards dichroic mirror 140. Thedichroic mirror 140 reflects the light 150 and allows mixing with thelight from diodes 130, 132 to form white light. The dichroic mirrors136, 138, 140 are angled or shaped to direct the white light in adirection towards a common optic axis 155. Each of the light sources122, 127 are configured with a predetermined polarization. In oneembodiment, the polarization of light source 142 is orthogonal to thepolarization of light source 144. Further, the light sources 142, 144are configured to alternately and sequentially emit light onto thecommon optic axis 155.

The light from the first light source 122 exits and passes through afly's eye lens 154. The fly's eye lens 154 is made up of an array oflenslets that have the effect of breaking the transmitted light intomany components and projecting them evenly over the field of view. Theresult is even, bright illumination without any reduction in lightintensity at the periphery of the projected light. Once the light leavesthe fly's eye lens 154, the light may pass through an optional condenserlens 156 that concentrates the light.

Next, the light passes through a focusing lens 158 that focuses thelight toward a mirror 160. The mirror 160 reflects and spreads the lightonto an imaging device 162. The light reflects off of the imaging device162 with a polarization that then substantially transmits through aprojection lens assembly 166 and out of the projector 120. This processis repeated in a sequential manner for the second light source.

In an exemplary embodiment, the imaging device 162 is a DMD. A DMD is anoptical semiconductor having several hundred thousand microscopicmirrors arranged in an array. The array of microscopic mirrors forms animage surface or plane that may then be projected. These surface mirrorscorrespond to pixels in the image being displayed. The mirrors areindividually rotated to either reflect the light into the projectionlens assembly 166 or reflect the light away (making it dark). Grey scalecolors are produced by toggling the microscopic mirrors very quickly.The amount of time the microscopic mirrors are reflecting intoprojection lens assembly 166 will determine the shade of grey.

As shown in FIG. 3 and FIG. 4, the imaging device 162 is arranged with afirst axis 170 that extends is substantially perpendicular from thecenter of the image surface of the DMD image device 162. The projectionlens assembly 166 is arranged on a second axis 168. The first axis 170and the second axis 168 are offset by a distance D such that mirror 160is arranged to reflect the light such that light 172 being reflected offof the imaging device 162 is at an angle that causes the light tointercept the projection lens assembly 166. In one embodiment, theprojector 120 includes an optional back reflection filter to reducespeckle.

The projector 120 also includes a feedback circuit 174. The feedbackcircuit 174 is electrically coupled to communicate with the first lightsource 122, the second light source 127 and the DMD imaging device 162.The feedback circuit 174 receives a modulation signal from the lightsources 122, 127 and from the DMD imaging device 162, and provides amodulation signal to the DMD imaging device 162. The modulation signalskeep the light sources 122, 127 and the DMD imaging device 162synchronized during operation. In other words, the feedback circuit 174ensures that the desired light source 122, 127 is emitting light thatcorresponds to the image projected through the projection lens assembly166.

With reference to FIG. 5, an exemplary embodiment of a feedback circuit174 is provided for use in a projector for projecting a 3D image, suchas projector 120 is generally shown. As shown in FIG. 5, the modulationrate (i.e., the on/off flashing rate) of the light sources 122, 127 ofFIG. 3 and FIG. 4 are being synchronized with the rotation speed of themirrors in the DMD imaging device 162. Thus, the first input devicebeing synchronized includes light sources 122, 127 and the second inputdevice is the DMD imaging device 162. A phase reference signal 173 isderived from the modulation rate of the light sources 122, 127, and thephase reference signal 173 is input to a PLL 188. While the modulationrate of the light sources 122, 127 is input into the feedback circuit174, input from the DMD imaging device 162, is simultaneously gatheredvia a sensor 180. In the embodiment shown in FIG. 5, the sensor 180measures the rotational speed of the mirrors in the DMD imaging device162. In alternate embodiments, the sensor 180 detects the modulationsignal driving the rotational speed of the mirrors in the DMD imagingdevice 162.

A phase feedback signal 181 that indicates the rotational speed of themirror in the DMD imaging device 162 is output from the sensor 180. Thephase feedback signal 181 output from the sensor 180 is adjustable via aprogrammable quiescence delay device 182 which is set based, forexample, on how fast the mirrors in the DMD imaging device 162 arespinning. The quiescence delay device 182 is an optional element of thefeedback circuit 174 which may be required to compensate for a delaythat is introduced due to the design or circuitry of the projector 120.When the quiescence delay device 182 is used, a DMD driver 190 thatcontrols the rotational speed of the mirrors in the DMD imaging device162 sends the quiescent delay device 182 a quiescence reference signal184 that is based on the current rotational speed of the mirrors in theDMD imaging device 162 such that any delay in the phase feedback signal181 may be eliminated to achieve synchronization accuracy. Additionally,an optional signal conditioner or amplifier 186 may be applied to thephase feedback signal 181 that is output from the quiescence delaydevice 182 prior to the phase feedback signal 181 being directed to thePLL 188.

The PLL 188 compares the phase reference signal 173 and the phasefeedback signal 181 to determine whether the rotation of the mirrors inthe DMD imaging device 162 are synchronized with the modulation rate ofthe light sources 122, 127. Based on the results of the comparison, thePLL 188 outputs a signal 192 to the DMD driver 190 causing the DMDdriver 190 to either increase or decrease the rotational speed of themirrors in the DMD imaging device 162.

Embodiments of the present invention provide for a feedback circuitcompatible with a 3D projector in a number of arrangements. Thedescription of the exemplary projector systems is meant to aid in theunderstanding of the application of the feedback circuit to a projectorsystem, and not to limit the invention. The present invention providesthe advantage of having synchronized components within a projector forensuring the accuracy of a projected 3D image. Embodiments of thepresent invention provide advantages in emitting a 3D image usable withpassive or active glasses.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the preferred embodiment to the invention had been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

What is claimed is:
 1. A projector system for a portable electronicdevice, the projector system comprising: a first input device; a secondinput device; a control device; a sensor; and a phase locked loop (PLL),the projector system configured for: creating a phase reference signalbased on a signal rate of the first input device; creating a phasefeedback signal based on the rotational speed of the second input deviceas measured by the sensor; comparing, by the PLL, the phase referencesignal and the phase feedback signal to determine whether the lightdevice and the second input device are synchronized; sending, by thePLL, a signal to the control device, the sending responsive todetermining that the first input device and the second input device arenot synchronized; and changing, by the control device, the rotationalspeed of the second input device resposive to the signal.
 2. Theprojector system of claim 1, wherein the projector system is furtherconfigured for applying a programmable delay to the phase feedbacksignal prior to the comparing by the PLL.
 3. The projector system ofclaim 1, wherein the projector system is further configured applying atleast one of a signal conditioner and a signal amplifier the phasefeedback signal prior to the comparing by the PLL.
 4. The projectorsystem of claim 1, wherein the projector system is further configuredfor modifying the phase reference signal prior to the comparing by thePLL.
 5. The projector system of claim 1, wherein the first input deviceis an imaging device.
 6. The projector system of claim 1, wherein thefirst input device is a light source.
 7. The projector system of claim1, wherein the second input device is a rotating polarizing beamsplitter (PBS).
 8. The projector system of claim 1, wherein the secondinput device is a digital mirror device (DMD) comprising rotatingmirrors.
 9. The projector system of claim 1, wherein the sensor is atachometer.
 10. A system comprising: a first input device; a secondinput device; a control device in communication with a digital mirrordevice (DMD) device, the control device configured for controlling arotational speed of the DMD device; a sensor in communication with theDMD device, the sensor configured for determining the rotational speedof mirrors within the DMD device and for outputting a phase feedbacksignal based on the rotational speed of the mirrors; a quiescence delaydevice electrically coupled to receive the phase feedback signal; and aphase locked loop configured for receiving a phase reference signal froma light source, for receiving the phase feedback signal from thequiescence delay device, for comparing the phase reference signal andthe phase feedback signal to determine whether the first input deviceand the second input device are synchronized, and for sending a firstsignal to the control device to change the rotational speed of themirrors responsive to determining that the light source and the DMDdevice are not synchronized.
 11. The system of claim 10, wherein thelight source and the DMD device are located on a three dimensional (3D)projector system for a portable electronic device.
 12. The system ofclaim 10, wherein the phase lock loop is configured for sending a secondsignal to the quiescence delay device in response to determining thatthe light source and the DMD device are not synchronized.
 13. The systemof claim 10, wherein the light source includes a first light source anda second light source, the first light source, the second light sourceand the DMD device being located on a 3D projector system for a portableelectronic device.
 14. A projector system for a portable electronicdevice, the projector system comprising: a first input device; a secondinput device; a polarizing beam splitter (PBS), the PBS configured torotate about a first axis; a tachometer operably coupled to measure arotational speed of the PBS; a control device configured for controllingthe rotational speed of the PBS; a phase locked loop (PLL) electricallycoupled to the tachometer; and a quiescence delay device electricallycoupled between the tachometer and the PLL, the quiescence delay deviceconfigured to delay the transmission of a phase feedback signal; whereinthe phase locked loop is configured for receiving a phase referencesignal from the first input device, for receiving the phase feedbacksignal from the quiescence delay device, for comparing the phasereference signal and the phase feedback signal to determine whether thefirst input device and the second input device are synchronized, and forsending a first signal to the control device to change the rotationalspeed of the PBS in response to determining that the first input deviceand second input device are not synchronized.
 15. The projector systemof claim 14 wherein the PLL is further configured to generate a secondsignal in response to determining that the first input device and thePBS are not synchronized, and the quiescence delay device changes adelay period of the phase feedback signal in response to receiving thesecond signal.
 16. The projector system of claim 15 wherein the lightsource comprises: a first light source emitting a first light along anoptical axis, the optical axis being substantially perpendicular to thefirst axis; a second light source emitting a second light along theoptical axis in a direction substantially opposite the first lightsource; and the PBS being disposed between the first light source andthe second light source.
 17. The projector system of claim 16 whereinthe image generator is arranged to receive the first light and thesecond light reflected with the PBS and configured to reflect the firstlight and the second light therefrom.